Conventionally, for example, as disclosed in Japanese Laid-Open Patent Application 4-121260, a brake force control apparatus which generates, when an emergency braking is required, a brake force greater than that generated in a normal time is known. The above-mentioned conventional apparatus comprises a control circuit which generates a drive signal corresponding to an operational speed of a brake pedal and a fluid pressure generating mechanism which generates a brake fluid pressure corresponding to the drive signal generated by the control circuit.
The control circuit determines that, when an operational speed of a brake pedal is less than a predetermined value, the brake pedal is not normally operated. In this case, the fluid pressure generating mechanism is controlled so that a brake fluid pressure corresponding to a brake pressing force is generated. Hereinafter, this control is referred to as a normal control. Additionally, the control circuit determines that, when an operational force of the brake pedal exceeds a predetermined value, an emergency braking is required by the driver. In this case, the fluid pressure generating mechanism is controlled so that a brake fluid pressure is maximized. Hereinafter, this control is referred to as a brake assist control. Thus, according to the above-mentioned conventional apparatus, a brake force corresponding to a brake pressing force can be generated in a normal time, and a large brake force can be immediately generated in an emergency. Thereby, an emergency braking can be performed and a rapid braking can be achieved even if the driver intends to perform an emergency braking but is unable to hold a large pressing force applied to a brake pedal.
Conventionally, in order to improve a driving stability, apparatuses for a vehicle which perform an antilock brake control (hereinafter, referred to as an ABS control), a vehicle stability control (hereinafter, referred to as ASC), a traction control (hereinafter, referred to as TRC) or a suspension attenuation force control are known. These apparatuses control a distribution of braking and driving forces, a distribution of rolling rigidity, a posture, and a distribution of a load to each wheel so as to improve a driving stability of the vehicle with respect to changes in a moving condition or a driving condition of the vehicle. Hereinafter, such an apparatus which improves a driving stability of a vehicle is referred to as a vehicle characteristic changing apparatus. By the above-mentioned conventional brake force control apparatus is provided to a vehicle together with the vehicle characteristic changing apparatus, a rapid braking can be achieved by performing a brake assist control and an improvement of a driving stability of a vehicle can be achieved so that safety of the vehicle is greatly improved.
However, in the vehicle having the above-mentioned vehicle characteristic changing apparatus, when an abnormality occurs in the vehicle characteristic changing apparatus, a driving stability is changed as compared to that of a normal time. Additionally, in the vehicle having the above-mentioned vehicle characteristic changing apparatus, when the brake assist control is performed as an emergency braking is required, a side force which can be generated by the vehicle is decreased and the driving stability is changed since a large brake force is generated rapidly. Accordingly, In the vehicle having both the above-mentioned vehicle characteristic changing apparatus and the brake force control apparatus, when the brake assist control is performed by the above-mentioned brake force control apparatus under a condition in which the vehicle characteristic changing apparatus malfunctions, an amount of change of a driving stability is increased as compared to a case in which the vehicle characteristic changing apparatus is normal and before the brake assist control is performed.